Read-only items are not supported correctly

With some cloud providers, there is the possibility that files or folders do not have write permission (for example, because the item was shared read-only).
We currently have no way to detect this early and notify the user about it when adding the vault in the main app. Also, in the `FileProviderExtension`, we could adjust the permissions accordingly so that the user does not have write permission in the Files app for these items (or the entire vault).
I think in our case it normally applies to the entire Vault, otherwise the user would have part of the Vault shared read-only and the rest of the Vault shared with full permission (which should be very unlikely).
To support the above scenario properly, changes need to be made in `CloudAccess` (more precisely in `CloudItemMetadata`).